1. Field of the Invention
The present invention relates to an offshore construction comprising a floating body, at least one anchor line for connecting the floating body to the sea bed, the anchor line being connected to the floating body via a flexible top part which is attached to a connector on the floating body, the connector comprising a housing having two mutually perpendicular pivot axes with an elongate tubular member extending in the anchor line direction and a latch mechanism for allowing movement of the anchor line towards the floating body and preventing movement in the direction of the sea bed.
2. Description of Related Art
Such an offshore construction is known from WO 00/78599 in which a mooring buoy is disclosed which is anchored to the seabed in which the anchor chains are attached to the buoy via a pivoting chain receiving tube comprising a chain stopper by which the chain can be tensioned. By providing two orthogonal pivot axes for the chain tensioner, chain wear can be reduced and the tension in the chain upon movement of the buoy is decreased.
In the known construction, however, the upper chain link will be fixed in position by the chain stopper device whereas the adjacent chain link can still move. Due to the high tension within the chain and the friction between the fixed link in the known chain hawser and the moveable link, which is attached to the link fixed by the chain stopper, this moveable link will be subject to bending fatigue which may lead to chain failure of the link within the hawser.
It therefore is an object of the present invention to provide an anchor line connector in which anchor line fatigue in general is reduced.
It is also object of the present invention to provide a chain connector in which chain link fatigue and in particular chain link fatigue through out of plane bending is reduced.
Thereto, the offshore construction according to the present invention is characterized in that the connector comprises a rod which can be accommodated within the tubular member, which rod is connected to the flexible top part, the rod comprising a broad head part for engaging with the latch mechanism and a lower part for engaging with the sidewall of the tubular member and having an attachment member for attaching to the flexible top part, the rod being longer than the tubular member such that the attachment member is situated outside of the tubular member.
The flexible part, which is at the top end of the anchor line, could be a chain part or could be a steel wire or a polyester rope part with a lug at its free end. In case of the chain part, the end chain link is connected to the rod, which can then be inserted into the tubular member and can be fixed in place with its broadened head part behind the latch mechanism. The attachment member remains located outside of the tubular member such that the chain link attached to the rod can freely move. When the chain part reaches a predetermined angle, the tubular member will pivot upon reaching a predetermined break out torque, such that it will swing and reduce the interlink angle at the position near the connector and, hence, out of plane bending of the chain links.
In one embodiment, the rod comprises a broadened middle part for engaging with the wall of the tubular member. The broadened middle part distributes the forces exerted via the chain part on the tubular member, more evenly along its length.
The connector according to the present invention can be designed by first determing the allowable stress range on the chain links for the life of the mooring system, based on a fatigue approach. Thereafter, the angular movement between the links corresponding to the allowable stress range is determined. This is based on experimental stress analysis. Next, the minimum length of the tubular member and rod according to the present invention is determined to ensure that the tubular member is rotated before the maximum allowable angle of the chain links is reached. In order to limit the length of the tubular member, low friction bushes for the pivot axis are preferred.
The perpendicular pivot or gimble arrangement provides articulation in orthogonal directions. The articulation in the transverse direction is needed to limit the pressure on the bushes of the pivot axis, which provide articulation in the radial direction, as well as avoiding fatigue failure in transversely orientated links.